Magnetorheological fluids are fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field. The fluids typically include ferromagnetic or paramagnetic particles dispersed in a carrier fluid. The particles become polarized in the presence of an applied magnetic field, and become organized into chains of particles within the fluid. The particle chains increase the apparent viscosity (flow resistance) of the fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid.
Magnetorheological fluids have been proposed for controlling damping in various devices, such as dampers, shock absorbers, and elastomeric mounts. They have also been proposed for use in controlling pressure and/or torque in brakes, clutches, and valves. Magnetorheological fluids are considered superior to electrorheological fluids in many applications because they exhibit higher yield strengths and can create greater damping forces.
Magnetorheological fluids are distinguishable from colloidal magnetic fluids or ferrofluids. In colloidal magnetic fluids, the particle size is generally between 5 and 10 nanometers, whereas the particle size in magnetorheological fluids is typically greater than 0.1 micrometers, usually greater than 1.0 micrometers. Colloidal magnetic fluids tend not to develop particle structuring in the presence of a magnetic field, but rather, the fluid tends to flow toward the applied field.
Some of the first magnetorheological fluids, described, for example, in U.S. Pat. Nos. 2,575,360, 2,661,825, and 2,886,151, included reduced iron oxide powders and low viscosity oils. These mixtures tend to settle as a function of time, with the settling rate generally increasing as the temperature increases. One of the reasons why the particles tend to settle is the large difference in density between the oils (about 0.7-0.95 g/cm.sup.3) and the metal particles (about 7.86 g/cm.sup.3 for iron particles). The settling interferes with the magnetorheological activity of the material due to non-uniform particle distribution. Often, it requires a relatively high shear force to re-suspend the particles.
A limitation of these magnetorheological fluids is that they are prepared with organic carrier fluids, such as oils, which can become polymerized, degrade, promote growth of bacteria and be flammable. In addition, organic carrier fluids can be incompatible with components of the device in which it is used. It would be advantageous to have magnetorheological fluids that do not include organic carrier fluids or which only include water-miscible organic solvents, to overcome the limitations of oil-based magnetorheological fluids.
Prior attempts at preparing water-based magnetorheological fluids used various thickening agents, such as xanthan gum and carboxymethyl cellulose as described in U.S. Pat. No. 5,670,077. These formulations can be difficult to mix, and tend to settle over time.
In addition to particle settling, another limitation of the fluids is that suspension agents such as silica and silicon dioxide tend to cause wear when they are in moving contact with the surfaces of various parts. It would be advantageous to have magnetorheological fluids that do not cause significant wear when they are in moving contact with surfaces of various parts. It would also be advantageous to have magnetorheological fluids using water-based solvent systems that are capable of being re-dispersed with small shear forces after the magnetic-responsive particles settle out. The present invention provides such fluids.